Well, I've been mucking about with a new character in a Master Mode-world since launch-time yesterday... and I've reached the conclusion that Master Mode isn't worth the effort.
After a mixture of playing it myself and reading up on it, the downsides of Master Mode are these:
# All monsters have more hit points and do more damage than Expert Mode.
# You lose ALL your money upon death.
Meaning that starting out with a fresh character is an exercise in tedium. Every step needed to advance takes so much longer because every monster is border-line lethal.
But wait! What are the upsides? They are:
# A character in a Master Mode-world gains an extra Item-slot, which can be combined with that Expert-mode item from the Wall of Flesh to gain a total of
seven item slots.
# Every boss drops a special golden statue, plus has a 25% chance to drop a Pet item!
So unless you're nuts about Terraria pets and accolades (or are an absolute masochist) there is NO reason to play Master Mode. Expert Mode is good enough.
But that's not the worst part: The worst part is that I dared venture into some Terraria-related forums... and it don't look good over there. People don't seem to like the update as much as they should (based on prior updates), for various reasons. Some of those reasons include:
# The new music doesn't match the old music. (My jury is still out on this one, but I will say that there are a surprisingly large number of new tracks.)
# The NPC 'happiness' mechanic. The major complaint is that it slows down the game too much... and I kinda have to agree with that. People wanted to have more control over the NPCs, but this seems like borderline bullshit.
# People are actually throwing about the tinfoil-hat theory that Re-Logic are purposefully making the game worse so that their upcoming game project gains more favoritism. This would make sense if Terraria was a AAA-product game on a tight timetable, but Terraria just turned 9 years old and is an indie-release, not a AAA-game.
# DAT Fucking Zoologist-NPC, a.k.a. the Furry. People just don't like him/her... because somehow some rumor got started that the Furry is ALSO a trans-character... and people are losing their shit because of sexual deviancy and mental illness getting featured in what suddenly now is "a children's game".
Also, the Zoologist keeps whining about trivial matters ("Stop pulling on my tail!") and sells
whips for some reason...
# "Terraria is STILL TOO EASY", because some players rolled something like seven new worlds on Master Mode and because they found several copies of key weapons early on one of them the whole game is still a cakewalk. Meanwhile I rolled the one world and my best weapon is a Mace that has some interesting properties, but I could upgrade it to a Fiercy Mace (just add 99 torches) to add extra damage plus burning! I still get fucking murdered just taking a peek outside my base though. Perspective is a helluva drug, yo!
Finally, some sage advice for people who want to start out with a fresh character:
# If you don't like all those gravestones your track record of failure leaves behind and try to mine them away, there's an extra string attached now that may make you reconsider. Who ya gonna call?
# In the Underground-layer you may come across Gold Chests out in the open. Go FULL PANIC MODE if you see them. If you have a pressure plate or some other item that can let you see wires, USE THEM. If the chest is wired you die just by opening it. In my experience numerous traps are attached to it PLUS a large explosive device that DOESN'T show up via wire-view. If there are no wires, then it's just a chest full of stuff, grab it!
# The Lifeform Analyzer reveals a lot more critters now than it did, including the <CHARNAME> of the Undead Merchant, Gnomes and some of the more uncommon-but-"Who-gives-a-fuck"-critters.
And on the topic of new bosses - AFAIK there are two of them, and both of them are connected to the Hallow. One is unique, one is a reskin of a previous boss. The unique boss is a "bullet-hell"-boss, and you'll die just by looking at the related fancy graphics. (Here's me lamenting the addition of a Winter-biome boss. ;( )
Conclusion: If you still have access to the 1.3.5.3-version of Terraria, HANG ON TO IT. But also, give 1.4 a try. You should quickly realize which version better caters to you.
(That said, OWLS.)